Conventionally, among direct injection internal combustion engines having a fuel injection valve (or an injector) for injecting fuel directly into the interior of the cylinder, one having a function of correcting the opening time of the fuel injection valve based on the pressure in the interior of the cylinder (or the in-cylinder pressure) is known (see, for example, patent document 1 listed later).
The reason why this function is adopted is that the pressure in the interior of the cylinder (the in-cylinder pressure) acts as a back pressure against the fuel injection valve. In this function, the in-cylinder pressure that changes in accordance with the running condition of the engine is calculated to correct the opening time of the fuel injection valve, thereby obtaining a desired fuel injection quantity.
However, in the above-described conventional art, the in-cylinder pressure is calculated, and then the fuel injection rate (i.e. the quantity of the fuel injected per unit time) is calculated based on the pressure difference between that in-cylinder pressure and the pressure of the fuel introduced to the fuel injection valve. Then, the opening time of the fuel injection valve is calculated based on the calculated fuel injection rate and the required fuel quantity. Thus, in the conventional art as such, no consideration has been given to changes in the start time of the fuel injection.
This point will be discussed in the following with reference to FIG. 14. FIG. 14 shows changing behavior of the fuel injection rate. In FIG. 14, the vertical axis represents the fuel injection rate, and the horizontal axis represents the time. In FIG. 14, waveform X and waveform Y of the fuel injection rate show changing behavior of the fuel injection rate for different in-cylinder pressures but the same rail pressure (i.e. the pressure of the fuel supplied to the fuel injection valve). Fuel injection rate waveform X represents the case in which the in-cylinder pressure is a reference in-cylinder pressure serving as a reference (for example, the pressure under the condition in an injector characteristics measuring benchmark test (e.g. 1 Mpa)), and fuel injection rate waveform Y represents the case with the engine in-cylinder pressure in the internal combustion engine in a running state (e.g. 8 Mpa).
As will be seen from FIG. 14, when the in-cylinder pressure increases, the start time of the fuel injection becomes earlier. If the start time of the fuel injection becomes earlier, the fuel injection quantity will increase.
FIG. 15 is a fuel injection rate changing behavior model serving as a model for the changing behavior of the fuel injection rate shown in FIG. 14. The inventors of the present invention modeled the changing behavior of the fuel injection rate shown in FIG. 14 as a trapezoid shown in FIG. 15. In FIG. 15, trapezoid X shown by a solid line is a model for fuel injection rate waveform X in FIG. 14, and trapezoid Y shown by a broken line is a model for fuel injection rate waveform Y in FIG. 14.
In FIG. 15, letting Q be the area of trapezoid X or the required fuel injection quantity, Qr be the area of trapezoid Y or the actual injection quantity, dQ1 be the variation in the fuel injection quantity due to the variation in the fuel injection rate between in the case of reference in-cylinder pressure and in the case of in-cylinder pressure of the engine (i.e. the area within trapezoid X above trapezoid Y in FIG. 15), and dQ2 be the variation in the fuel injection quantity due to the variation in the start time between in the case of reference in-cylinder pressure and in the case of in-cylinder pressure of the engine (i.e. the area within trapezoid Y on the left side of trapezoid X in FIG. 15), the actual injection quantity Qr can be represented by the following formula (1)Qr=Q−dQ1+dQ2  (1)
Therefore, a command value for attaining desired fuel injection is represented by the following formula (2).Q=Qr+dQ1−dQ2  (2)
In FIG. 15, letting A be the length of the upper base of trapezoid X, B be the length of the lower base of trapezoid X, Q′ be the height of trapezoid X (i.e. the fuel injection rate in the case of reference in-cylinder pressure), q′ be the height of trapezoid Y and C be the length of the portion of the upper base of trapezoid Y that overlaps trapezoid X, dQ1 is represented by the following formula (3)                                                         dQ1              =                              Q                -                q                                                                                        =                              Q                -                                                      (                                          B                      +                      C                                        )                                    ⁢                                                            q                      ′                                        /                    2                                                                                                                          =                              Q                -                                                      (                                          B                      +                                              (                                                                                                            Aq                              ′                                                        /                                                          Q                              ′                                                                                +                                                                                    B                              ⁡                                                              (                                                                                                      Q                                    ′                                                                    -                                                                      q                                    ′                                                                                                  )                                                                                      /                                                          Q                              ′                                                                                                      )                                                              )                                    ⁢                                                            q                      ′                                        /                    2                                                                                                                          =                                                                    (                                          1                      -                                                                        q                          ′                                                /                                                  Q                          ′                                                                                      )                                    ⁢                  Q                                +                                                      (                                          A                      -                      B                                        )                                    ⁢                                      (                                                                  q                        ′                                            -                                              Q                        ′                                                              )                                    ⁢                                                            q                      ′                                        /                                          Q                      ′                                                                                                                              (        3        )            
In the conventional art, dQ2 in formula (2) is not taken into consideration, and corrected fuel injection rate is obtained by the relationship represented by the following formula (4).Qr=Qq′/Q′  (4)
Thus, in the conventional art, dQ1 is represented by the following formula (5).                                                         dQ1              =                              Q                -                Qr                                                                                        =                                                (                                      1                    -                                                                  q                        ′                                            /                                              Q                        ′                                                                              )                                ⁢                Q                                                                        (        5        )            
Equation (5) lacks the second term in equation (3), and therefore, no consideration has been given to an error corresponding to this term in the conventional art.
In addition, when the rail pressure is low, the change in the fuel injection rate after the start of the fuel injection is moderate. Therefore, the gradient of the left edge of the trapezoid is small and the value (A–B) is large. Thus, in the case that the rail pressure is low, influence of the value (A–B) is significant, and therefore, a large error will be introduced if dQ1 is obtained from equation (5).    [Patent Document 1] Japanese Patent Application Laid-Open No. 9-256886    [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-54889